1. Field of the Invention
The present invention relates to an IC socket for establishing electrical connections with a semiconductor package, suitably used in LSI tests, such as function tests and burn-in tests.
2. Description of Related Art
The progress in integration degree and function of LSIs (Large Scale Integrated Circuit) due to the advance in the semiconductor technology is accompanied by the fact that an increasing number of external terminals are required for signal inputs and outputs of an LSI. Additionally, an increasing number of terminals are required for testing an LSI due to the recent tendency that functions integrated within the LSI have become more complex. It is desirable that the number of terminals used only for LSI tests should be as small as possible since such terminals are not used by users in the actual use. Although various efforts have been made to decrease the number of test terminals, including commonly using terminals for test and actual use and serially inputting and outputting test signals, the number of test terminals is currently on the increase.
The number of terminals mountable onto an LSI package is physically limited, and this makes it difficult to connect all the contact pads of an LSI chip with external terminals of an LSI package. This tendency is particularly remarkable for SIPs (System In Package). Therefore, test terminals are not given high priority in providing connections with external terminals of the LSI package, since test terminals are not necessary in the actual use of the LSI; however, the lack of test terminals is undesirable for improving the reliability of LSIs.
The inventor considers that one approach for providing a desired number of test terminals is to use an IC package including both of a ball grid array (BGA) and a land grid array (LGA); such IC package is referred to as BGA/LGA hybrid package, hereinafter. The ball grid array, composed of an array of solder balls, is used to establish physical and electrical connection with a printed circuit board, and the land grid array, composed of an array of lands, is only used in the LSI test. Such package structure allows providing a desired number of test terminals for an IC package with a reduced package size.
One issue is to establish electrical connections with both of the ball grid array and land grid array. In general, IC sockets are used for establishing electrical connections with BGA packages or LGA packages (BGA/LGA packages) in conducting functional tests and burn-in tests of LSIs. A variety of IC sockets have been developed to provide secure electrical connections between LSIs and terminals of IC sockets without damaging the LSIs. For instance, Japanese Laid Open Patent Application No. Jp-A2003-123923 (hereinafter, the '923 application) and the corresponding U.S. Patent Application Publication No. 2005/070134 disclose an IC socket for testing a BGA package. The disclosed IC socket is composed of a socket body and a plurality of contacts to be connected with solder balls of a BGA package. The socket body has a plurality of mounting-holes into which the contacts are inserted. Each of the mounting holes has a through hole which is penetrated in the height direction of the body, and a support hole for supporting the contact. Each of the contacts has an upright piece extending through the through-hole and a support piece extending from the upright piece to be inserted into the through-hole from a top surface side of the socket body. A contact portion to be brought into contact with the solder ball is formed at a tip end portion of the upright piece and the support piece is extending from a proximal end portion of the upright piece.
Additionally, Japanese Laid Open Patent Applications No. Jp-A2002-246131 (hereinafter, the '131 application) and Jp-A Heisei 11-86992 (hereinafter, the '992 application) disclose IC sockets provided with plungers to be connected with solder balls, the plungers being biased against solder balls with coil springs. Specifically, the '131 application discloses an IC socket in which contact pins are disposed through a socket body to provide electrical connections between solder balls of a BGA package and a printed circuit board. Each contact pin includes first and second plungers. The first plunger is electrically connected to one of the solder balls and the second plunger is electrically connected to a terminal provided on the printed circuit. Between the first plunger and the second plunger provided are a cylindrical body and an elastic member. The cylindrical body is struck against the first plunger on one end, and slidably accommodates the second plunger at the other end. An elastic member is disposed between the cylindrical body and the second plunger, and the first and second plungers are biased away from each other by the elastic member.
The '992 application discloses a spring connector used in an IC socket which provides electrical connections between a solder ball of a BGA package and a printed circuit board. The spring connector is provided with a cylindrical body and a plunger. One end of the cylindrical body is closed with a bottom plate, and a protrusion formed of solder is provided on the bottom plate. The cylindrical body is made of conductive material. The other end of the cylindrical body is open, and the plunger is slidably inserted into the cylindrical body through the opening. A spring is provided in the cylindrical body, biasing the plunger toward the solder ball of the BGA packages.
Furthermore, Japanese Laid Open Patent Application Jp-A2001-196144 (hereinafter, referred to as the '144 application) discloses an IC socket for an LGA package. The IC socket disclosed in the '144 application is provided with a housing plate with a concave portion, and an array of terminals provided in the concave portion. The terminals are arranged at the positions corresponding to lands of the land grid array package. Each of the terminals has a bent spring plate and a pair of side plates. The spring plate is allowed to expand and shrink in the thickness direction of the housing plate. When the spring plate is expanded, the upper end of the spring plate protrudes from the surface of the concave portion of the housing plate. The side plates are provided to sandwich the spring plate and to extend in the thickness direction of the housing plate. The spring plate is provided with protrusions which are slidably contacted with the side plates.
Nevertheless, the IC socket structures disclosed in the above-mentioned applications are not adapted to the BGA/LGA hybrid package. The contact points of the respective external terminals of the IC socket structures disclosed in the above-mentioned applications are coplanar on the same plane; however, such structures are not preferable for the BGA/LGA hybrid package. In the BGA/LGA hybrid package, the contact points of the ball grid array are not coplanar with those of the land grid array. The use of any of the IC socket structures disclosed in the above-mentioned applications for establishing electrical connections with a BGA/LGA hybrid package results in poor electrical connections or excessive contact pressure.